1. Field of the Invention
Embodiments of the present invention generally relate to optical waveguide sensors, and more particularly to a fiber Bragg grating optical waveguide sensors that dynamically senses strain induced by a stimuli acting upon a transduction mechanism.
2. Description of the Related Art
A fiber Bragg grating (FBG) is an optical element that is formed by a photo-induced periodic modulation of the refractive index of an optical waveguide's core. An FBG element is highly reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength that is referred to as the Bragg wavelength. Other wavelengths pass through the FBG without reflection. The Bragg wavelength itself is dependent on physical parameters, such as temperature and strain, that impact on the refractive index. Therefore, FBG elements can be used as sensors to measure such parameters. After proper calibration, the Bragg wavelength acts is an absolute measure of the physical parameters.
One way of using fiber Bragg grating elements as sensors is to apply strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a fiber Bragg grating element. For example, U.S. Pat. No. 6,016,702, issued Jan. 25, 2000, entitled “High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments” by inventor Robert J. Maron discloses an optical waveguide sensor in which a compressible bellows is attached to an optical waveguide at one location while a rigid structure is attached at another. A fiber Bragg grating (FBG) is embedded within the optical waveguide between the compressible bellows and the rigid structure. When an external pressure change compresses the bellows the tension on the fiber Bragg grating is changed, which changes the Bragg wavelength.
Another example of using fiber Bragg grating elements as pressure sensors is presented in U.S. Pat. No. 6,422,084, issued Jul. 23, 2002, entitled “Bragg Grating Pressure Sensor” by Fernald, et al. That patent discloses optical waveguide sensors in which external pressure longitudinally compresses an optical waveguide having one or more fiber Bragg grating. The optical waveguide can be formed into a “dog bone” shape that includes a fiber Bragg grating and that can be formed under tension or compression to tailor the pressure sensing characteristics of the fiber Bragg grating. Another fiber Bragg grating outside of the narrow portion of the dog bone can provide for temperature compensation.
While the foregoing pressure sensing techniques are beneficial, those techniques may not be suitable for all applications. Therefore, fiber Bragg grating techniques suitable for dynamically sensing varying parameters such as pressure and strain would be useful. Also useful would be fiber Bragg grating techniques that provide for both static and dynamic measurements of parameters.